eMedicine Specialties > Ophthalmology > Extraocular Muscles

Esotropia and Exotropia, A-patterns

James L Plotnik, MD, FACS, Consulting Staff, Department of Ophthalmology, Division of Pediatric Ophthalmology, Arizona Pediatric Eye Specialists

Updated: Dec 18, 2008

Introduction

Background

Horizontal deviations can be divided into 2 broad categories, as follows: esotropias and exotropias. Esotropia designates a convergent horizontal strabismus; exotropia designates a divergent horizontal strabismus. Horizontal deviations are subdivided further into comitant and incomitant deviations (also referred to as concomitant and noncomitant, respectively). Comitant refers to an ocular deviation that does not vary with the direction of gaze; incomitant describes an ocular deviation that varies with the direction of gaze.

Vertically incomitant describes a horizontal misalignment of the eyes in which the magnitude of the horizontal deviation differs in upgaze when compared to downgaze. The following are common patterns seen in vertically incomitant horizontal deviations: A-patterns, V-patterns, and, less commonly, Y-patterns; lambda-patterns; and X-patterns. These patterns are named using letters of the alphabet whose shapes have visual similarities to the ocular motility patterns that they describe. Vertical incomitance may be seen with both esotropias and exotropias.

The term A-pattern designates a vertically incomitant horizontal deviation in which there is more convergence in midline upgaze and less convergence (increased divergence) in midline downgaze. By convention, an A-pattern is not considered to be clinically significant unless the distance measurements of the ocular deviation in midline upgaze (25° above primary gaze) and midline downgaze (25° below primary gaze) differ by at least 10 prism diopters. The term A-pattern is used because the vertical lines that comprise the letter A converge near the apex of the letter and diverge at the bottom of the letter. The appearance of the letter A reflects the clinical situation.

An A-pattern esotropia is an inward deviation of the visual axes in which there is more inward deviation of the eyes in midline upgaze than in midline downgaze. An A-pattern exotropia is an outward deviation of the visual axes in which there is more divergence of the eyes in midline downgaze than in midline upgaze. Lambda-pattern is used to describe a subtype of A-pattern strabismus. In this situation, little change occurs in the amount of ocular deviation from midline upgaze to primary position, but increased divergence occurs between primary position and downgaze.

Pathophysiology

Oblique muscle dysfunction

With significant A-patterns, version testing usually reveals superior oblique muscle overaction. The tertiary abduction effect of the superior oblique muscle is believed to produce the A-pattern. The abducting force is greatest in downgaze, the superior oblique's primary field of action, causing an increased relative divergence of the eyes in downgaze.

Generally, 2 types of oblique muscle dysfunction are associated with an A-pattern, primary superior oblique muscle overaction and secondary superior oblique muscle overaction. Primary superior oblique muscle overaction refers to overaction of the muscle with no identifiable etiology. The exact cause of the overaction remains unclear. Several hypotheses of the overaction exist. Why some individuals manifest oblique muscle dysfunction is unknown. Neurologic and mechanical hypotheses have been proposed. Inferior oblique muscle paresis is a rare entity that can cause secondary overaction of the ipsilateral superior oblique muscle.

Horizontal rectus muscle dysfunction

As a proposed cause or contributing factor in the development of an A-pattern strabismus, horizontal rectus muscle dysfunction could explain why A-pattern strabismus may occur without apparent superior oblique muscle overaction.

According to this theory, an A-pattern esotropia would be due to underaction of the lateral rectus muscles; an A-pattern exotropia would be due to underaction of the medial rectus muscles. Electromyographic studies of patients with A-pattern strabismus support this theory.

Vertical rectus muscle dysfunction

Abnormally functioning vertical rectus muscles have been proposed as a cause or contributing factor in the development of A-patterns. This theory is based on the tertiary adduction action produced by the vertical rectus muscles in their field of action to produce the A-pattern.

According to this theory, overaction of the superior rectus muscles would result in increased adducting effect (increased convergence) in upgaze. Underaction of the inferior rectus muscles would result in decreased adduction (decreased convergence) in downgaze, thereby producing an A-pattern.

Abnormalities of extraocular muscles or globe torsion

A-pattern strabismus may be related to the complex interplay of the ocular muscles and orbital soft tissues. Abnormalities in the location of the orbital connective tissue sleeves (which act as mechanical muscle pulleys) can cause incomitant deviations, simulating overaction of the superior oblique muscle. The heterotopic muscle pulleys, probably a superiorly displaced lateral rectus muscle pulley, may be the etiology of A-patterns, and the oblique muscles may be normal.

Torsion of the globe may be the etiology of horizontally incomitant strabismus. Globe torsion may be due to abnormal oblique muscle function or loss of fusion. Globe rotation alters the relationship of the extraocular muscles and/or extraocular muscle pulleys. By altering the vertical alignment of the horizontal rectus muscle insertions, these muscles can act as partial elevators or depressors. Conversely, by altering the horizontal alignment of the vertical rectus muscle insertions, these muscles can act as partial abductors or adductors. The vector forces would be changed in both magnitude and direction. According to this theory, intorsion of the globe alters the vector forces of the rectus muscles, causing an A-pattern.

Anatomic abnormalities of facial structure

Although their exact significance remains unclear, certain facial characteristics have been noted in some individuals with vertically incomitant strabismus.

A-pattern esotropia has been associated with both flat lid margins and eyes in which lateral canthi are higher than medial canthi, producing a mongoloid appearance.

A-pattern exotropia has not been associated with the appearance of lid fissures.

Frequency

United States

The frequency of A-pattern deviations among the general population is unknown. Among individuals with strabismus, the prevalence of an A-pattern varies among different studies, ranging from as low as 4.5% to as high as 36%. The ratio of A-pattern esotropia to A-pattern exotropia is approximately 2.2:1.

Mortality/Morbidity

In some individuals, the presence of an A-pattern strabismus may be insignificant and only cosmetically bothersome to the patient and the patient's family. Conversely, the misalignment of the visual axes of the 2 eyes may interfere with the patient's ability to fuse and develop normal binocular vision. It also may cause diplopia in children and adults. Abnormal vertical head postures may develop to place the eyes in a position of minimal deviation to restore single binocular vision.

A child with an A-pattern esotropia may be orthotropic in primary gaze, and even able to fuse in downgaze, but manifest a significant esotropia in upgaze. Although this situation may be functionally satisfactory, the cosmetic appearance of the inward deviation in upgaze may be quite disconcerting to the parents.

Conversely, a child with an A-pattern exotropia may appear aligned in upgaze but not be able to fuse in downgaze. Parents may be unaware of the eye misalignment because the eyes may appear straight when they look at their child. Although this may be cosmetically acceptable, this is functionally undesirable.

Race

No racial predilection exists.

Sex

No sexual predilection exists.

Age

No age predilection exists.

Clinical

History

Inward or outward deviation of the eyes is the most common presenting problem.

• The vertical variation in the magnitude of the horizontal deviation may not be obvious to the parents or the patient.
• A head tilt (chin up or down) may be present, as the individual adopts a head posture that allows the eyes to remain in the position of minimal deviation. This compensatory maneuver minimizes diplopia and allows binocular viewing.

Physical

Measurements of the amplitude of horizontal deviation are obtained by prism and alternate cover testing in primary position and, then, with the eyes 25° in upgaze and 25° in downgaze. Measurements should be made while the patient wears proper refractive correction and fixates on an accommodative target at distance. Note any underaction and overaction of the oblique muscles on versions and any compensatory abnormal head posture. Examine the palpebral fissure configuration (presence of a mongoloid appearance).

• Clinical findings of A-pattern esotropia
  • Esotropia increases in midline upgaze and decreases in midline downgaze.
  • Eyes may be straight in downgaze and primary gaze.
  • Patients may demonstrate a chin-up posture, a compensatory maneuver that requires the eyes to be in downgaze for straight-ahead viewing. This posture places the eyes in the position of gaze where less inward deviation (more divergence) of the eyes occurs, possibly allowing single binocular vision.
• Clinical findings of A-pattern exotropia
  • Exotropia increases in midline downgaze and decreases in midline upgaze.
  • Eyes may be straight in upgaze and primary gaze.
  • Patients may demonstrate a chin-down posture, a compensatory maneuver that requires the eyes to be in upgaze for straight-ahead viewing. This posture places the eyes in the position of gaze where less outward deviation (more convergence) of the eyes occurs, possibly allowing single binocular vision.
• Patients with A-patterns may manifest signs of superior oblique overaction, including overdepression in adduction, incyclotorsion of the involved eye(s), and/or associated vertical strabismus.
  • Patients may demonstrate a tight superior oblique muscle on forced duction testing.
  • Most patients with congenital overaction of the superior oblique muscles do not manifest subjective complaints of torsion; instead, they manifest objective evidence of intorsion by indirect ophthalmoscopy. This may be most noticeable in downgaze.

Causes

See Pathophysiology.

Differential Diagnoses

Workup

Laboratory Studies

• No special diagnostic studies, laboratory tests, or imaging studies help in evaluating and/or treating these patients.

Imaging Studies

• Refer to those articles on esotropia and exotropia listed in the Differentials section for the indications for neuroimaging of patients with strabismus.

Treatment

Medical Care

As with comitant esotropias and exotropias, nonsurgical means can be employed to alleviate the ocular deviation.

• Significant refractive errors should be corrected to aid in ocular alignment.
• Prisms and orthoptic training may be attempted when indicated. However, the incomitant nature of A-pattern deviations makes these modalities less effective, and, when an A-pattern is present, they are rarely beneficial.

Surgical Care

Surgically treat only A-patterns of clinical significance. When planning surgery, the surgeon should recognize that the primary and reading positions are functionally the most important positions of gaze; direct efforts at minimizing deviations in these positions should occur. When planning for strabismus surgery to minimize an A-pattern strabismus, assess superior oblique muscle overaction and inferior oblique underaction. Most patients with large A-patterns have significant oblique muscle dysfunction.

• Vertical displacement of the horizontal rectus muscle insertions is recommended when the A-pattern is small (<20 prism diopters) and there is little or no apparent overaction of the oblique muscles.
  • To reduce an A-pattern, the medial rectus muscles are moved toward the direction of vertical gaze where the convergence is greater (upward); this loosens the muscle in upgaze and tightens the muscle in downgaze, which results in a relative weakening of adduction in upgaze and strengthening in downgaze.
  • The lateral rectus muscles are moved toward the direction of vertical gaze where the divergence is greater (downward); this has the effect of tightening the muscle in upgaze and loosening the muscle in downgaze, resulting in the relative weakening of abduction in downgaze and strengthening in upgaze.
  • The amount of vertical displacement of the horizontal recti is one-half to a full tendon width. A one-half tendon width vertical transposition on 2 horizontal rectus muscles eliminates approximately 15-20 prism diopters of A-pattern deviation.
  • Some surgeons vary the amount of vertical displacement, depending on the size of the A-pattern (ie, more displacement for larger A-patterns); others perform the same amount of displacement on all patients.
  • The vertical transposition of the horizontal recti usually is combined with a resection or recession of the horizontal recti to correct the deviation in primary position.
  • Vertical displacement of the horizontal recti has little effect on primary position eye alignment or on ocular torsion.
  • Binocular surgery for A-pattern esotropia
    • Bilateral medial rectus muscle recession for the amount of deviation in primary gaze with upward displacement of both medial recti
    • Alternatively, bilateral lateral rectus muscle resection for the amount of deviation in primary gaze with downward displacement of both lateral recti
  • Binocular surgery for A-pattern exotropia
    • Bilateral lateral rectus muscle recession for the amount of deviation in primary gaze with downward displacement of both lateral recti
    • Alternatively, bilateral medial rectus muscle resection for the amount of deviation in primary gaze with upward displacement of both medial rectus muscles
  • Monocular surgery for A-patterns
    • The medial rectus muscle is displaced upward, and the ipsilateral lateral rectus muscle is displaced downward. This surgery usually is combined with the appropriate recession-resection procedure.
    • The medial rectus is weakened (decreased adduction), and the lateral rectus muscle is strengthened (increased abduction) in upgaze. The opposite occurs on downgaze.
    • The transposition has no net vertical effect in primary position.
• Weakening of the superior oblique muscles is indicated when overaction of the superior oblique muscles is present and the A-pattern is large.
  • Various procedures have been used. Weakening procedures include tenotomy, tenectomy, graded recession, or lengthening with a silicone expander.
  • When overaction of the superior oblique muscles is associated with a clinically significant horizontal deviation, horizontal muscle surgery should be performed at the same time as the oblique muscle surgery. A bilateral recession, bilateral resection, or recess-resect procedure is performed on the horizontal rectus muscles to correct the horizontal misalignment in primary position.
  • Bilateral superior oblique tenotomies correct large A-patterns associated with superior oblique overaction. The average amount of A-pattern corrected by this surgery varies from 23-45 prism diopters of A-pattern correction in downgaze (less divergence) postoperatively.
    • The amount of weakening induced by the superior oblique tenotomy can be graded. A tenotomy closer to the origin causes more weakening than a tenotomy closer to the insertion. For mild-to-moderate A-patterns, bilateral superior oblique tenotomy with disinsertion of the posterior seven eighths of the superior oblique tendon, leaving the anterior fibers intact, is advocated. This technique selectively weakens the vertical and abduction functions of the muscle but has minimal effect on intorsion. The likelihood of a postoperative cyclovertical deviation is decreased.
    • Most authors agree that little eso shift (less exodeviation) occurs in upgaze, but the result of bilateral superior oblique tenotomies on primary gaze is controversial. Some have found no significant alteration in primary position alignment postoperatively. Therefore, some surgeons do not alter the amount of horizontal muscle surgery performed.
    • Others believe that the loss of abducting forces from this surgery results in an eso shift (less exodeviation) in primary position of 10-15 prism diopters. These surgeons adjust the amount of horizontal muscle surgery performed to compensate for the anticipated change in primary position alignment.
  • The superior oblique can be weakened using a silicone tendon expander; a synthetic material is used to lengthen (weaken) the superior oblique tendon.
    • A superior oblique tenotomy is performed, and a silicone implant is inserted between the cut ends of the tendon.
    • The amount of weakening can be altered by varying the length of the expander. Typically, a 4-7 mm piece of No. 240 silicone band is used as an expander. Longer pieces are used to correct greater amounts of overaction.
    • This surgery is beneficial because it allows for graded lengthening of the superior oblique tendon and is less prone to overcorrection and is reversible.
    • In a recent study by Sharma, bilateral weakening of the superior oblique tendon via tenotomy with insertion of a 6 mm silicone expander produced a mean correction of 23 prism diopters of A-pattern, which corrected 95% of the preexisting A-pattern.
  • Bilateral superior oblique recession has been used in small studies by Romano and Drummond, respectively, to correct an A-pattern.
    • This technique allows the superior oblique to be weakened in a graduated manner without the tendency to cause overcorrections.
    • In a study by Sharma, bilateral recession of the superior oblique produced a mean correction of 30.7 prism diopters of A-pattern.
    • Although some studies report it as effective, this technique is not commonly used to eliminate an A-pattern.
  • Symmetrical horizontal transposition of the vertical rectus muscle insertions
    • The vertical rectus muscles are secondary adductors. By altering their site of insertion, the adducting forces can be modified.
    • To reduce an A-pattern esotropia, the superior recti are moved temporally 5-7 mm to lessen their adducting effect (less convergence) in upgaze.
    • To reduce an A-pattern exotropia, the inferior recti are moved nasally 5-7 mm, thereby enhancing their adducting effect (more convergence) in downgaze.
    • This surgery is not commonly used because most surgeons have found it to be ineffective and unpredictable.
  • Although not widely used, bilateral inferior oblique advancement has been advocated for the treatment of A-pattern esotropia.
    • This technique consists of disinserting the inferior oblique muscle, passing it under the lateral rectus muscle, and reattaching it to the sclera 2 -3 mm superior to the lateral rectus and approximately 8 mm posterior to its insertion.
    • For esotropia, this technique is combined with horizontal muscle surgery.
    • In a study by Goldstein, the average correction of A-pattern was 23 prism diopters (range 4-33 prism diopters) with bilateral inferior oblique advancement.
  • Slanting of the muscle insertions has been used successfully to treat A-pattern esotropia in the absence of superior oblique overaction. The recessed medial recti are reattached to the sclera with the superior border of the muscle reattached 3 mm posterior to the insertion of the lower border of the muscle. This procedure preferentially weakens the superior aspect of the muscle more than the lower aspect of the muscles. Slanting the muscle insertion in this manner decreases the muscle's ability to adduct the eye in upgaze, decreasing the A-pattern.

Medication

No effective medical treatments exist.

Follow-up

Further Outpatient Care

• Patients should have routine follow-up examinations to determine if the eyes are straight.

Complications

• If not treated promptly, amblyopia can occur.

Prognosis

• Prognosis is favorable for cosmetic appearance. The patient's age at the time of treatment determines the success for binocularity.

Patient Education

• Early detection and prompt referral are important.

Miscellaneous

Medicolegal Pitfalls

• Prompt detection and treatment is necessary to prevent development of amblyopia.

Special Concerns

• In patients with good stereoacuity and the ability to fuse preoperatively, even a slight postoperative superior oblique paresis can cause bothersome cyclovertical diplopia by disrupting binocularity and the ability to fuse. Cautiously approach superior oblique tenotomy or tenectomy in these patients.
  • In particular, patients with an intermittent exotropia and an A-pattern tend to have binocular fusion and are at high risk for developing this complication.
  • Additionally, caution should be used when considering superior oblique weakening procedures, such as tenotomy or tenectomy, for A-pattern strabismus in the presence of inferior oblique overaction. Weakening of the superior oblique may result in a postoperative V-pattern.
• In the aforementioned situations, vertical transposition of the horizontal recti, controlled weakening of the superior oblique using a silicone tendon expander, superior oblique recession, or a posterior superior oblique tenotomy are preferred to decrease the likelihood of inducing symptomatic diplopia postoperatively.

References

1. Biedner B, Rothkoff L. Treatment for 'A' or 'V' pattern esotropia by slanting muscle insertion. Br J Ophthalmol. Sep 1995;79(9):807-8. [Medline].

2. Breinin GM. Electromyography: a tool in ocular and neurologic diagnosis. II. Muscle palsies. Arch Ophthalmol. 1957;57:165.

3. Brown HW. Symposium; strabismus; vertical deviations. Trans Am Acad Ophthalmol Otolaryngol. Mar-Apr 1953;57(2):157-62. [Medline].

4. Campion GS. Symposium: the A and V patterns in strabismus. Clinical picture and diagnosis. Trans Am Acad Ophthalmol Otolaryngol. May-Jun 1964;68:356-62. [Medline].

5. Chen J, Mai G, Deng D. Clinical features and surgical treatment of A-pattern exotropia. Yan Ke Xue Bao. Sep 2004;20(3):163-7. [Medline].

6. Clark RA, Miller JM, Rosenbaum AL, Demer JL. Heterotopic muscle pulleys or oblique muscle dysfunction?. J AAPOS. Feb 1998;2(1):17-25. [Medline].

7. Diamond GR, Parks MM. The effect of superior oblique weakening procedures on primary position horizontal alignment. J Pediatr Ophthalmol Strabismus. Jan-Feb 1981;18(1):35-8. [Medline].

8. Drummond GT, Pearce WG, Astle WF. Recession of the superior oblique tendon in A-pattern strabismus. Can J Ophthalmol. Oct 1990;25(6):301-5. [Medline].

9. Fierson WM, Boger WP 3rd, Diorio PC, et al. The effect of bilateral superior oblique tenotomy on horizontal deviation in A-pattern strabismus. J Pediatr Ophthalmol Strabismus. Nov-Dec 1980;17(6):364-71. [Medline].

10. Goldstein JH. Inferior oblique advancement for "A" pattern esotropia. Ophthalmic Surg. Jul 1986;17(7):412-4. [Medline].

11. Hugonnier R. Introduction a l'etude des problemes poses par les strabismes avec syndromes A et V. J Pediatr Ophthalmol Strabismus. 1965;2:11-14.

12. Jampolsky A. Bilateral anomalies of the oblique muscles. Trans Am Acad Ophthalmol Otolaryngol. Nov-Dec 1957;61(6):689-98; discussion 698-700. [Medline].

13. Jin YH, Sung KR, Kook MS. The immediate effect of bilateral superior oblique tenotomy on primary position horizontal binocular alignment. Binocul Vis Strabismus Q. Spring 1999;14(1):33-8. [Medline].

14. Kushner BJ. The role of ocular torsion on the etiology of A and V patterns. J Pediatr Ophthalmol Strabismus. Sep-Oct 1985;22(5):171-9. [Medline].

15. Miller JE. Vertical recti transplantation in the A and V syndromes. Arch Ophthalmol. 1960;64:39-43.

16. Parks MM. Commentary on superior oblique tenotomy for A-pattern strabismus in patients with fusion. Binocular Vision. 1988;3:39.

17. Romano P, Roholt P. Measured graduated recession of the superior oblique muscle. J Pediatr Ophthalmol Strabismus. Jul-Aug 1983;20(4):134-40. [Medline].

18. Rubin SE, Nelson LB, Harley RD. A complication in weakening the superior oblique muscle in A-pattern exotropia. Ophthalmic Surg. Feb 1984;15(2):134-5. [Medline].

19. Scott WE, Arthur BW. Current approaches to superior oblique muscle surgery. In: Focal Points. 1988;VI (module 3):1-2.

20. Sharma P, Khokhar S, Thanikachalam. Evaluation of superior oblique weakening procedures. J Pediatr Ophthalmol Strabismus. Jul-Aug 1999;36(4):189-95. [Medline].

21. Shin GS, Elliott RL, Rosenbaum AL. Posterior superior oblique tenectomy at the scleral insertion for collapse of A-pattern strabismus. J Pediatr Ophthalmol Strabismus. Sep-Oct 1996;33(5):211-8. [Medline].

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24. Wright KW, Min BM, Park C. Comparison of superior oblique tendon expander to superior oblique tenotomy for the management of superior oblique overaction and Brown syndrome. J Pediatr Ophthalmol Strabismus. Mar-Apr 1992;29(2):92-7; discussion 98-9. [Medline].

Keywords

A-patterns, A-pattern esotropia, A-pattern exotropia, A-pattern strabismus, alphabet pattern strabismus, vertically incomitant horizontal deviation

Contributor Information and Disclosures

Author

James L Plotnik, MD, FACS, Consulting Staff, Department of Ophthalmology, Division of Pediatric Ophthalmology, Arizona Pediatric Eye Specialists
James L Plotnik, MD, FACS is a member of the following medical societies: American Academy of Ophthalmology and American Association for Pediatric Ophthalmology and Strabismus
Disclosure: Nothing to disclose.

Medical Editor

Michael J Bartiss, OD, MD, Medical Director, Ophthalmology, Family Eye Care of the Carolinas
Michael J Bartiss, OD, MD is a member of the following medical societies: American Academy of Ophthalmology, American Academy of Pediatrics, American Association for Pediatric Ophthalmology and Strabismus, American College of Surgeons, and North Carolina Medical Society
Disclosure: Nothing to disclose.

Pharmacy Editor

Simon K Law, MD, PharmD, Assistant Professor of Ophthalmology, Jules Stein Eye Institute; Chief of Section of Ophthalmology Surgical Services, Department of Veterans Affairs Healthcare Center, West Los Angeles
Simon K Law, MD, PharmD is a member of the following medical societies: American Academy of Ophthalmology, American Glaucoma Society, and Association for Research in Vision and Ophthalmology
Disclosure: Nothing to disclose.

Managing Editor

J James Rowsey, MD, Former Director of Corneal Services, St Luke's Cataract and Laser Institute, Florida
J James Rowsey, MD is a member of the following medical societies: American Academy of Ophthalmology, American Association for the Advancement of Science, American Medical Association, Association for Research in Vision and Ophthalmology, Florida Medical Association, Pan-American Association of Ophthalmology, Sigma Xi, and Southern Medical Association
Disclosure: Nothing to disclose.

CME Editor

Lance L Brown, OD, MD, Ophthalmologist, Affiliated With Freeman Hospital and St John's Hospital, Regional Eye Center, Joplin, Missouri
Disclosure: Nothing to disclose.

Chief Editor

Hampton Roy Sr, MD, Associate Clinical Professor, Department of Ophthalmology, University of Arkansas for Medical Sciences
Hampton Roy Sr, MD is a member of the following medical societies: American Academy of Ophthalmology, American College of Surgeons, and Pan-American Association of Ophthalmology
Disclosure: Nothing to disclose.

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